It depends on a lot of factors, but generally, he is right about a mirror being somewhat ineffective as a laser defense. It'd have to be an absolutely perfect mirror, which doesn't exist. If it would exist, you could also make a laser by using a perfect mirror, which's power would only be limited by the power you are able to supply. It stands to reason that any laser can always beat a "defense mirror" of same quality if it has the necessary power, since it can deliver the photons onto a much smaller area than it uses to focus the beam. Hence if you tax a mirror to the max, a defending mirror with the same properties will always be defeated. Sure, with a perfect mirror, the maximum power you can put into the beam is infinity, but mathematically it still works out. Since a perfect mirror is just as hypothetical as infinite power, we might as well think it to the end...